I have watched the video for double knitting here and it seems very doable, however it didn't cover how you start a piece of work if you want it to be double knitting from start to finish or even how you'd transition from single to double knitting. If someone could explain:

1) How to cast on and begin double knitting from the beginning of a piece of work?

2) If you are starting a piece in single knitting and going to double, how do you go about doing the transition?

I found double knitting infuriatingly frustrating... but I managed to cast on and do a couple rows before deciding it was not for me.

You will cast on holding both colours of yarn. When you have finished casting on, you go back and make sure they are lined up properly. You can do this by pulling one over the other to line it up... Ill use red and black as my 2 colours.

It should always be RED BLACK RED BLACKthis can reversed to BLACK RED BLACK RED Basically you just want to make sure that the colours next to each other are opposite... so there should be no RED BLACK BLACK RED....

Once you have the cast on, you can watch amy's video and follow what she does for the general knitting. It is a shame she doesn't have the cast on method shown here...

Make sure to keep your yarn balls seperate. Things can get tangled quickly.

I take both threads/yarn colors
and with BOTh strands in hand, i do whatever cast on I want (long tail for me)
then I have equal numbers of stitches side by side
yeah I have to switch some Red black black Red to Red Black Red Black
the first row is the ONLY one that is hard
once they are in line, I find it is easiest to think in terms of following the pattern as ONe sided, BUT, to do the stitch you do color one knit, then color two Purl
so if the color pattern is red red red black red red red black red red red black
you Kred Pblack Kred Pblack Kred Pblack, Kblack Pred, Kred Pblack Kred Pblack Kred Pblack Kred Pblack Kred Pblack Kred Pblack, Kblack Pred, Kred Pblack Kred Pblack Kred Pblack
it sounds hard, and at first it is
then it gets easy and it can be done
I did the "they call them pirates" hat in Double knitting
turned out GREAT (the second time I did it)
I also did the fake Isles hat, that one came out GREAT
I still wear it, its SO warm

ecb

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Make this world a Better, and more beautiful place, that you have been in it
Edward Bok
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Good judgement comes from experience
but Experience comes from Bad Judgment ecb

I do double knitting all the time. I do the same cast on as the other two have mentioned.
I also think of a pattern as one sided, but do the stitches as "stitch sets" as described by ecb.
If you are going to knit part of a pattern in single knit and then switch, I would say the easiest way is to just hold both yarns together for the row before you begin the double knit. If you have to have all one color on that row (not the mix you get if you hold two colors together), then use an extra strand of your main color yarn and join the contrasting color after that row, then as you go back the other way pick up the stitches as mentioned above (k1 "red", p1 "black", repeat until end of row). Remember that on your reverse side rows you have to do this in reverse (p1 black, k1 red). I almost always do this for hats knit in the round so I don't have to worry about reverse side rows.

It seems so clearly explained yet I am having a bit of trouble visualizing it. I am going to have to just give this ago, and I'll use your information and way of keeping the order of colours and which stitch to do when written down in chart form. I'll let you know how it goes.